The present invention relates to a pneumatic tire, more particularly to a pneumatic tire including a circumferential main groove having a cross sectional shape specified to reduce generation of cracks in the vicinity of the groove bottom.
FIG. 5 shows a partial cross sectional view of a tread portion in the vicinity of a circumferential groove of a conventional pneumatic tire, in which an imaginary line shows a cross sectional profile line of the groove just after vulcanization of the tire. In the tread portion of the pneumatic tire, a tensile strain generates in the vicinity of a bottom “b” of a circumferential groove “a” by shrinkage of a rubber due to lowering of temperature after the vulcanization. Such a strain will cause cracking at the groove bottom “b”. In particular, cracking is easy to occur at arc-like corner portions “d” between the groove bottom “b” and groove walls “c”, since the tensile strain largely acts on the corner portions “d”.
Further, as shown in FIG. 5B. a shoulder land portion B1 located on a tread edge side to which a load is easy to apply during cornering, is generally formed into a larger size than a middle land portion B2. Therefore, the amount of rubber shrinkage mentioned above is larger at the shoulder portion, so cracking occurs more easily at an arc-like corner portion “do” located on the shoulder land portion B2 side.
In order to suppress such a cracking at the groove bottom, it is proposed to use a rubber having a high degree of swelling in the tread portion of the tire. However, the rubber having a high degree of swelling tends to deteriorate the rolling resistance and the steering stability of the tire, since the rubber hardness tends to decrease.
In order to improve the cracking resistance at grooves, JP-A-2006-027465 proposes a circumferential groove having a specific cross sectional shape.
It is an object of the present invention to provide a pneumatic tire having an improved resistance to cracking which occurs at a groove bottom and in the vicinity thereof.
This and other objects of the present invention will become apparent from the description hereinafter.